In electronic systems, it is often desirable to provide arrays of devices assembled to form systems or subsystems. Typical arrays are composed of DRAM, SRAM, ROM, PROM or other memory devices. Access and control to the individual devices within these arrays are established by signals which contain the necessary information to access a particular device within the array of devices. In order to maintain proper control and access of the devices within the array, it is often necessary to buffer the control signals before they are applied to the devices in the array.
Past approaches to providing the necessary buffers for array control signals include using a separate buffering device for each of the control signals required. Therefore, if the control signal is comprised of N individual signals then the array would require a buffering device for each of the N signals in the control signal. This adds devices to the array which require board space and power.
Also, if field installation of additional banks to the array is desired, i.e. field expansion of memory size, then buffers for each potential expansion bank must be included in the base assembly. For example, if the base array includes only bank zero, designing the system to accommodate the field installation of banks 1-7, requires including buffers for each of the potential expansion banks 1-7, even though the expansion buffers are not in the base system. This makes the base array larger and more expensive than necessary for its initial application.
Providing proper drive current for the control signals in an array is sometimes also needed in an array. For example, in DRAM memory systems with multiple banks of devices, drivers are required for the address, mode, row-address-strobe (RAS), and column-address-strobe (CAS) inputs. Each DRAM device and its associated interconnecting printed wiring circuit presents capacitance to each signal input. For proper DRAM function each input signal must be driven with a fast and strong driver to charge and discharge the total line capacitance in a short time. Large DRAM arrays require redundant, high speed, high power drivers to achieve acceptable performance.
The traditional solution of using discrete buffers to buffer signals in arrays has numerous disadvantages. The individual discrete buffers occupy board space, and consume power and therefore generate localized heat buildup in the system. Discrete buffers in arrays with many signals buffered in the array also produce large localized power and ground current spikes when simultaneously switching. The additional signal conductor lengths required in the printed circuit board to route the signals to the buffers adds capacitance to the signal path. In systems with expansion capability, the additional buffers must be included in the original base system. Therefore, the base system includes additional buffers that are unnecessary to its initial operation adding to the cost of the base unit. Finally, including separate discrete buffers in the array adds to the array's cost.